Bio 120 Lecture 1

1. The Origin of Life: How Did Life Begin?

The earliest life on Earth is thought to have originated about 3.5 to 4 billion years ago. However, life did not appear instantly—it evolved from non-living chemical compounds through a gradual, multi-step process.

Step 1: Abiotic Synthesis (Non-Living Chemistry Forms Organic Molecules)

Before life began, the Earth’s atmosphere did not contain oxygen like it does today. Instead, it was composed of gases such as:

  • Methane (CH₄) – A simple carbon-containing molecule.

  • Ammonia (NH₃) – A nitrogen-containing gas.

  • Hydrogen (H₂) – The simplest and lightest element.

  • Water Vapor (H₂O) – Water existed mainly as steam due to Earth's heat.

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These molecules reacted due to energy sources such as lightning, volcanic heat, and ultraviolet (UV) radiation from the Sun. These reactions led to the formation of organic molecules like amino acids and nucleotides—the building blocks of proteins and DNA/RNA.

Miller-Urey Experiment: Simulating Early Earth’s Chemistry

In 1953, two scientists, Stanley Miller and Harold Urey, performed a famous experiment to test whether organic molecules could form from simple chemicals under early Earth-like conditions.

Experiment Setup:

  • They created a closed system with gases similar to early Earth’s atmosphere (methane, ammonia, hydrogen, and water vapor).

  • They applied electric sparks (simulating lightning).

  • After a few days, they found amino acids and other organic molecules in the water.

Significance:

  • This experiment proved that the building blocks of life could form naturally from non-living chemicals.

  • It supports the idea that life may have started in Earth’s early oceans (often called the "primordial soup").

Step 2: Formation of Polymers (Complex Molecules Like Proteins and DNA)

  • Once amino acids and nucleotides were present, they linked together to form longer molecules:

    • Proteins (built from amino acids)

    • RNA/DNA (built from nucleotides)

These molecules stored genetic information and catalyzed chemical reactions, bringing life closer to forming.

Step 3: Self-Replication (The Ability to Copy Itself)

  • Some RNA molecules were able to replicate—meaning they could make copies of themselves.

  • This was crucial because life needs a way to pass information from one generation to the next.

Step 4: Formation of the First Cells (Protocells)

  • Some molecules, particularly lipids, naturally form membranes in water.

  • These membranes enclosed RNA and proteins, forming protocells (primitive, cell-like structures).

  • Protocells could grow, divide, and evolve—leading to the first true living cells.

Professor’s Commentary

  • He describes the first cells as "tiny drops of the ancient ocean," encapsulated in membranes.

  • Life requires liquid water—it cannot function in steam or ice, which is why scientists search for liquid water when looking for life on other planets.

  • The early Earth was violent: meteor impacts, volcanic eruptions, and intense radiation bombarded the planet, but despite this, life managed to form and survive.

2. Timeline of Life on Earth

Life evolved gradually over billions of years.

Event

Time (Million Years Ago)

Description

Formation of Earth

4,600

Earth forms from dust and gas.

First Oceans Form

4,300

Water vapor cools and condenses.

First Prokaryotic Cells Appear

3,500

Simple, single-celled bacteria-like organisms.

Oxygen Accumulates (Great Oxygenation Event)

2,700

Cyanobacteria produce oxygen via photosynthesis.

First Eukaryotic Cells Appear

2,200

Complex cells with a nucleus evolve.

Multicellular Life Evolves

1,000

Some cells stick together, forming larger organisms.

Cambrian Explosion

530

Most modern animal groups appear.

Plants and Fungi Colonize Land

475

Life moves from water to land.

Professor’s Commentary

  • Oxygen was originally toxic to most early life. The first organisms to use it evolved aerobic respiration, leading to more complex life forms.

  • During the Permian period (~300 million years ago), oxygen levels reached 30%, allowing giant insects (e.g., dragonflies the size of hawks) to exist.

3. Prokaryotes vs. Eukaryotes: Understanding Cell Types

Prokaryotes (Bacteria & Archaea)

  • Small, simple, unicellular organisms with no nucleus.

  • DNA floats freely in the cytoplasm.

  • Have a cell membrane, ribosomes, and sometimes a cell wall.

  • Examples: E. coli, Streptococcus, Cyanobacteria.

Eukaryotes (Protists, Plants, Fungi, Animals)

  • Larger, more complex cells with a nucleus and organelles.

  • Organelles like mitochondria and chloroplasts help cells generate energy.

  • Examples: Humans, trees, fungi, algae.

Endosymbiosis: How Eukaryotic Cells Evolved

  • Mitochondria and chloroplasts were once independent bacteria.

  • A larger cell engulfed them, but instead of digesting them, they formed a partnership.

  • Evidence:

    • Mitochondria and chloroplasts have their own DNA.

    • They replicate independently inside cells.

    • They have double membranes, suggesting they were once separate bacteria.

Professor’s Commentary

  • He calls mitochondria "orange jelly beans" and chloroplasts "green edamame" floating inside cells.

  • This "cell-eating event" was an evolutionary milestone, allowing cells to use oxygen efficiently.

4. How Bacteria Shaped the Planet

Bacteria in Ecosystems

  • Decomposers: Break down dead organisms, recycling nutrients.

  • Nitrogen Fixers: Convert nitrogen from the air into forms plants can use.

  • Pathogens: Some bacteria cause diseases (e.g., cholera, tuberculosis).

Bacterial Shapes

  • Cocci – Spherical bacteria (e.g., Streptococcus).

  • Bacilli – Rod-shaped bacteria (e.g., E. coli).

  • Spirilla – Spiral-shaped bacteria (e.g., Lyme disease bacterium).

Professor’s Commentary

  • Bacteria help regulate Earth’s carbon, nitrogen, and phosphorus cycles.

  • Many bacteria are beneficial (gut bacteria aid digestion), despite how they’re marketed as bad in cleaning products.

Final Takeaways

  • Life originated from non-living chemistry, evolving through gradual steps.

  • Bacteria and archaea were Earth’s first life forms and still dominate today.

  • Oxygen revolutionized life, making complex organisms possible.

  • Endosymbiosis led to eukaryotic cells and eventually multicellular life.

  • Bacteria are crucial to life on Earth, influencing ecosystems and human health.